1. Field of the Invention
This invention relates to a sensing system using optical fibers to measure the distribution of some measurands, such as temperature, pressure and etc., distributed along an optical fiber's longitudinal direction.
2. Prior Art
A technique called Optical Time Domain Reflectometry (OTDR) has been popularly used for the distributed sensing of the measurands such as temperature, pressure and so on that are distributed along the optical fiber's longitudinal direction. This technique utilizes the property of an optical fiber that an introduced pulse light having a given wave length that enters an optical fiber from of end of eventually returns to that end with a part of the light experiencing a certain degree of back scattering at a given point along the longitudinal direction of the fiber. This allows one to determine the location of the point as a function of the time required for the measured light to come back and to calculate the loss of the light at the given point of the optical fiber by the difference between the intensity of the incident light and that of the reflected light. Conversely, if the loss of light introduced at a point in an optical fiber varies as a function of ambient temperature, pressure and other variable factors, the physical quantities such as temperature and pressure which are specific to the point can be determined by observing the variation of the loss of the light at the point.
The above described method is, however, accompanied by certain disadvantages. The ratio of back scattering light intensity relative to the light introduced into the optical fiber of an instrument using this technique is normally too small. So is the S/N ratio, due to the fact that the introduced light is a pulse light and therefore the quantity of the light that can be detected per unit time is limited. The dynamic range of instrumentation of such an instrument is very small. Consequently, more than several seconds or, in some instances, more than several minutes will be required for one instrumentation, as it involves tens of thousands of averaging operations when a high accuracy of measurement is essential. Since a commercially available wave length instrument has a dynamic range of 17 dB to 20 dB, for higher dynamic ranges, there have been proposed sensing systems using optical fibers that utilize a high-power semiconductor laser device that can generate a light of approximately 1 W or a Nd:YAG laser device having an output of several watts. However, such a system inevitably involves a large cost for the light source and the risk of generating in the optical fiber nonlinear optical effects that, in turn, cause noises in the system.
It is therefore an object of the present invention to provide a sensing system using optical fibers that utilize a relatively large quantity of light per unit time and offer an improved S/N ratio and a widened dynamic range of instrumentation.